Antenna for implantable medical devices

ABSTRACT

The disclosure describes examples of antennas used for communication with an implantable medical device (IMD). As one example, the IMD includes a housing configured to house communication circuitry within an internal side of the housing, and a planar antenna, having a curved structure, that is stacked on an external side of the housing and coupled to the communication circuitry. As another example, the IMD includes a housing configured to house communication circuitry within an internal side of the housing and an antenna having a curved structure formed on an external side of the housing and coupled to the communication circuitry. A resonant frequency of the antenna is based on a dielectric constant of tissue surrounding the antenna when the IMD is implanted, and a current distribution of the antenna is in-phase in opposite sides of the antenna.

The application is a continuation of U.S. patent application Ser. No.16/515,449, filed Jul. 18, 2019, the entire contents of which areincorporated by reference herein.

TECHNICAL FIELD

The disclosure relates to medical device communication and, moreparticularly, to an antenna for an implantable medical device.

BACKGROUND

Various implantable medical devices have been clinically implanted orproposed for therapeutically treating or monitoring one or morephysiological and/or neurological conditions of a patient. Such devicesmay be adapted to monitor or treat conditions or functions relating toheart, muscle, nerve, brain, stomach, endocrine organs or other organsand their related functions. Advances in design and manufacture ofminiaturized electronic and sensing devices have enabled development ofimplantable devices capable of therapeutic as well as diagnosticfunctions such as pacemakers, cardioverters, defibrillators, biochemicalsensors, implantable loop recorders, and pressure sensors, among others.Such devices may be associated with leads that position electrodes orsensors at a desired location, or may be leadless with electrodes orsensors integrated into the device housing. These devices may have theability to wirelessly transmit data either to another device implantedin the patient or to another device located externally of the patient,or both.

Although implantation of some devices requires a surgical procedure,other devices may be small enough to be delivered and placed at anintended implant location in a minimally invasive manner, such as by apercutaneous delivery catheter or transvenously. By way of illustrativeexample, implantable miniature sensors have been proposed and used inblood vessels to measure directly the diastolic, systolic and mean bloodpressures, as well as body temperature and cardiac output of a patient.As one example, patients with chronic cardiovascular conditions,particularly patients suffering from chronic heart failure, may benefitfrom the use of implantable sensors adapted to monitor blood pressures.As another example, subcutaneously implantable monitors have beenproposed and used to monitor heart rate and rhythm, as well as otherphysiological parameters, such as patient posture and activity level.Such direct in vivo measurement of physiological parameters may providesignificant information to clinicians to facilitate diagnostic andtherapeutic decisions. In addition, miniaturized pacemakers that may beimplanted directly within a patient's heart, with or without the needfor leads to position electrodes, have been proposed, built, and adaptedto provide pacing and other electrical therapy to the patient.

These example devices communicate with external devices or other devicesimplanted within the patient. For example, the devices transmitinformation indicative of the sensed data. The devices receiveinformation such as therapy and sensing parameters and other informationthat defines modes of operation.

SUMMARY

The disclosure describes medical devices, systems, and associatedtechniques, structures, and assemblies including or involving an antennathat may be used to provide communications between medical devices andone or more other device(s). In some examples, the medical devices thatinclude these antennas may be small devices and may have been implantedwithin the patient under the skin or even relatively deeper within thepatient, for example implanted on or within the heart of a patient.

As described in more detail, this disclosure describes examples of anantenna having a curved (e.g., closed or partly open) structure withfeed points that cause for an in-phase current distribution at aresonant frequency. Due to the in-phase current distribution, theantenna described in this disclosure may not be sensitive to tissueconductivity. Accordingly, the antenna can be formed external to theimplantable medical device such as a planar antenna that is stacked onthe housing. In some examples, when implanted, the antenna may be indirect contact with patient tissue or very thin insulation may separatethe antenna from the patient tissue.

In one example, the disclosure describes an implantable medical device(IMD) comprising a housing configured to house communication circuitrywithin an internal side of the housing, and a planar antenna, having acurved structure, that is stacked on an external side of the housing andcoupled to the communication circuitry.

In one example, the disclosure describes a method of manufacturing animplantable medical device (IMD), the method comprising forming ahousing configured to house communication circuitry within an internalside of the housing, stacking a planar antenna, having a curvedstructure, on an external side of the housing, and coupling the planarantenna with the communication circuitry.

In one example, the disclosure describes an implantable medical device(IMD) comprising a housing configured to house communication circuitrywithin an internal side of the housing, and an antenna having a curvedstructure formed on an external side of the housing and coupled to thecommunication circuitry. A resonant frequency of the antenna is based ona dielectric constant of tissue surrounding the antenna when the IMD isimplanted, and a current distribution of the antenna is in-phase inopposite sides of the antenna.

The summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the systems, device, and methods describedin detail within the accompanying drawings and description below.Further details of one or more examples of this disclosure are set forthin the accompanying drawings and in the description below. Otherfeatures, objects, and advantages will be apparent from the descriptionand drawings, and from the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a conceptual drawing illustrating an example medical devicesystem in conjunction with a patient according to various examplesdescribed in this disclosure.

FIG. 2 is a diagram of an implantable medical device in accordance withone or more examples described in this disclosure.

FIG. 3 is a diagram illustrating an example of an antenna in accordancewith one or more examples described in this disclosure.

FIG. 4 is a conceptual diagram illustrating an example of currentdistribution on the example antenna illustrated in FIG. 3.

FIG. 5 is a diagram illustrating another example of an antenna inaccordance with one or more examples described in this disclosure.

FIG. 6 is a graph illustrating an example of return loss of an antennain accordance with one or more examples described in this disclosure.

FIG. 7 is a flowchart illustrating an example method of manufacturing inaccordance with one or more examples described in this disclosure.

FIG. 8 is a block diagram illustrating an example of stackedarchitecture of an antenna on an implantable medical device.

DETAILED DESCRIPTION

This disclosure generally relates to examples of antennas that areformed on a housing of a medical device. The example antennas may takeform of a curved (e.g., closed or partly open) structure (e.g.,rectangle, circular, elliptical, etc.) with feed points for a current toflow through the example antennas. As described in more detail, thecombination of the feed points and the curved structure cause thecurrent to distribute in-phase in the example antennas. For instance,the direction of the current on one side of the curved structure is thesame as the direction of the current on the opposite side of the curvedstructure.

Moreover, the example antennas may not be as sensitive to conductivityof patient tissue because the antenna provides a lower impedance path ofthe current as compared to impedance through tissue. For instance, dueto the in-phase distribution of the current in the example antennas, thecurrent is more likely to flow through the antenna than through tissue.Because the example antennas may not be sensitive to the conductivity ofpatient tissue, it may be possible to form the example antennas on thehousing of the medical device such that when the medical device isimplanted, an antenna of the medical device is in contact with thepatient tissue.

Patient tissue tends to have a relatively high dielectric constant,especially as compared to air or polymers used to insulate existingantennas. For a given resonant frequency, a size of an antenna isinversely proportional to the dielectric constant (e.g., for a lowdielectric constant, the size of the antenna needs to be bigger for thesame resonant frequency as an antenna surrounded by a higher dielectricconstant). Because the example antennas described in this disclosure canbe in contact with the patient tissue, the example antennas described inthis disclosure may be in an environment with relatively high dielectricconstant and can therefore have a relatively small size. The exampleantennas may be planar antennas (e.g., virtually no volume) and formedon the housing of the medical device such that when the medical deviceis implanted, the antenna is in contact with the patient tissue.

For instance, some other antennas that cannot be in contact with tissue(e.g., due to sensitivity to the conductivity of tissue) are formedwithin a header and surrounded by a polymer with relatively lowdielectric constant. This header is then connected to the medicaldevice. Due to the relatively low dielectric constant, the size of theseother antennas become too large to be formed on a side of the housingwith very little volume.

Accordingly, this disclosure describes examples of antennas that can beformed on an external side of the medical device such that the antennasutilize minimum volume. This results in an overall smaller medicaldevice, as compared to other medical devices having a header that housesthe antenna, which is beneficial for implantation.

As described above, one of the reasons that the example antennasdescribed in this disclosure may be smaller than antennas in a header isbecause the example antennas are in contact with patient tissue whenimplanted. In this disclosure, “in contact” may refer to the antennasbeing surrounded by material having a dielectric constant set by thepatient tissue such that the resonant frequency of the antennas is afunction of a dielectric constant of the patient tissue. Stated anotherway, the functionality and operational characteristics of the antennasis based on a dielectric constant of the patient tissue. For instance,in some examples, the antennas may be in direct contact with the patienttissue. However, it may be possible that in some examples, a protectivecoating is applied to the antenna to protect the antenna from damage.Even in such a scenario, the antenna may be considered as being “incontact” with patient tissue because the dielectric constant of thetissue is determinative of the resonant frequency of the antenna.

FIG. 1 is a conceptual drawing illustrating an example of somecomponents of a medical device system 100 in conjunction with a patient102 according to various examples described in this disclosure. Thesystems, devices, and techniques described in this disclosure provideimplantable medical devices (IMDs) that may include an antenna arrangedin a manner further described throughout this disclosure, tocommunicatively link the IMD(s) with one or more external device(s) 110,and/or to each other, as further described below. System 100 may includea single IMD, such as IMD 101, implanted in patient 102. As one example,IMD 101 may be inserted just under skin in an out-patient procedure. IMD101 may be an insertable cardiac monitor (ICM), as one example.

Also, for ease of illustration, system 100 includes a plurality of IMDs.However, the techniques described in this disclosure do not require theuse of a plurality of IMDs. In some examples, system 100 may includeonly one IMD (e.g., IMD 101). Also, the example techniques are notnecessarily limited to implantable medical devices, and may be extendedto other devices such as wearable medical devices such as where theantenna may still be in contact with the skin of the patient (e.g.,glucose sensor/pump), non-medical wearable devices (e.g., monitoringdevices worn on externally that monitor steps, pulse rate, etc.), andother devices including cell phone.

System 100 in some examples includes a plurality of IMDs, for examplesome combination of IMD 101, IMD 103, and/or IMD 105, as furtherdescribed below. In various examples, at least one of the IMDs in system100 includes an antenna configured as described in this disclosure.Also, in some examples, there may only be on IMD (e.g., IMD 101) Forpurposes of this disclosure, knowledge of cardiovascular anatomy ispresumed, and details are omitted except to the extent necessary ordesirable to explain the context of the techniques of this disclosure.Although the example techniques are described with respect to the heart,the example techniques are not limited to cardiac therapy. For instance,the example techniques described in this disclosure may be extended tonon-cardiac medical devices that provide communication (e.g., devicesfor pain stimulation, brain stimulation, pelvic stimulation, spinalstimulation, etc. and devices such as implanted drug pumps, and thelike).

As illustrated in FIG. 1, system 100 includes IMD 101 which may be aninsertable cardiac monitor (ICM) capable of sensing and recordingcardiac electrogram (EGM) (also referred to as an electrocardiogram,ECG, or EKG when external electrodes are placed on the skin) signalsfrom a position outside of heart 104 via electrodes (not shown in FIG.1). In some examples, IMD 101 includes or is coupled to one or moreadditional sensors, such as accelerometers, that generate one or moresignals that vary based on patient motion and/or posture, blood flow, orrespiration. Examples of IMD 101 may monitor a physiological parameterindicative of patient state, such as posture, heart rate, activitylevel, and/or respiration rate. IMD 101 may be implanted outside of thethorax of patient 102, e.g., subcutaneously or submuscularly, such asthe pectoral location illustrated in FIG. 1. In some examples, IMD 101may take the form of a Reveal LINQ® ICM, available from Medtronic plc,of Dublin, Ireland. In other examples, IMD 101 may be a pacemaker, e.g.,configured to sense electrical activity of heart 104, and/or to deliverpacing therapy, e.g., bradycardia pacing therapy, cardiacresynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy,and/or post-shock pacing, to heart 104, e.g., via intracardiac orextracardiac leads (not shown), and/or a cardioverter/defibrillatorconfigured to detect tachyarrhythmias and deliver anti-tachyarrhythmiashocks to heart 104 via the one or more leads.

In various examples, IMD 101 is configured to wirelessly communicatewith one or more external device(s) 110 as illustratively shown in FIG.1 by communication link 112. External device(s) 110 may be a computingdevice, e.g., used in a home, ambulatory, clinic, or hospital setting,to wirelessly communicate with IMD 101. For example, external device(s)110 may be a patient monitor, such as the MyCareLink™ patient monitor,or a programming instrument, such as the CareLink SmartSync™ system,available from Medtronic Inc., a subsidiary of Medtronic plc of Dublin,Ireland. In another example, external device(s) 110 may be a mobilecomputing device such as a smartphone, tablet, smartwatch, or otherwearable or portable device. External device(s) 110 may, for example,include a mobile application, such as MyCareLink Heart™ mobile app,available from Medtronic Inc., a subsidiary of Medtronic plc of Dublin,Ireland, that enables external device(s) 110 to communicate with IMD101. External device(s) 110 may be coupled to a remote patientmonitoring system, such as CareLink™ Network, available from MedtronicInc., a subsidiary of Medtronic plc, of Dublin, Ireland. Externaldevice(s) 110 may be, as examples, a programmer, external monitor, orconsumer device, e.g., smart phone. External device(s) 110 may be usedto program commands or operating parameters into IMD 101 for controllingthe functioning of IMD 101. External device(s) 110 may be used tointerrogate IMD 101 to retrieve data, including device operational dataas well as physiological or neurological data accumulated in memory ofIMD 101. The interrogation may be automatic, e.g., according to aschedule, or in response to a remote or local user command. One or moreof these external device(s) 110 may also be referred to as an“instrument” or as a group of instruments.

Examples of communication techniques used by IMD 101 and externaldevice(s) 110 are not limited to any particular communication techniqueor communication protocol, and in some examples include tissueconductance communication (TCC) or RF telemetry, which may be an RF linkestablished via Bluetooth®, WiFi, or medical implant communicationservice (MICS). IMD 101 may utilize an antenna arranged as described inthis disclosure, or an equivalent thereof, to perform the communicationsassociated with IMD 101, in order to provide any of the features and toperform any of the functions ascribed to IMD 101.

In various examples, one or more of the IMDs in FIG. 1, may include theantenna arranged in accordance with the examples of antenna described inthis disclosure, and any equivalents thereof, to facilitate thecommunications with the one or more IMDs of system 100, and/or betweenthe one or more IMDs 101, IMD 103, IMD 105, and/or external device(s)110. In various examples, monitoring and/or delivery of therapy by IMD101 may be provided in conjunction with the features and functionsprovided by IMD 105. In some examples, IMD 105 may engage in wirelesscommunications between IMD 105 and one or more other IMD(s) 101 and/orIMD 103 to facilitate coordinated activity between IMD 105 and these oneor more other IMD(s). The wireless communication may by via TCC ofradio-frequency (RF) telemetry and may be one-way communication in whichone device is configured to transmit communication messages and theother device is configured to receive those messages, or two-waycommunication in which each device is configured to transmit and receivecommunication messages.

System 100 may also include an intracardiac pacing device IMD 105, insome examples. In the illustrated example, IMD 105 is implanted in theright-ventricle of patient 102, e.g., internal to the heart 104 ofpatient 102. In some examples, one or more IMDs (not specifically shownin FIG. 1) similar to IMD 105 may additionally or alternatively beimplanted within other chambers of heart 104 or attached to the heartepicardially. IMD 105 may be configured to sense electrical activity ofheart 104, and/or to deliver stimulation therapy such as pacing therapy,e.g., bradycardia pacing therapy, cardiac resynchronization therapy(CRT), anti-tachycardia pacing (ATP) therapy, and/or post-shock pacing,to heart 104. IMD 105 may be attached to an interior wall 108 of heart104 via one or more fixation mechanisms that penetrate the tissue. Asshown in FIG. 1, the fixation mechanisms may secure IMD 105 to thecardiac tissue and retain an electrode (e.g., a cathode or an anode) onthe housing of IMD 105 in contact with the cardiac tissue. In additionto delivering pacing pulses, IMD 105 may be capable sensing electricalsignals using the electrodes carried on the housing of IMD 105. Theseelectrical signals may be electrical signals generated by cardiac muscleand indicative of depolarizations and repolarizations of heart 104 atvarious times during the cardiac cycle.

In various examples, IMD 105 is configured to wirelessly communicatewith one or more external device(s) 110 as illustratively shown in FIG.1 by communication link 112. For instance, IMD 105 may communicate withexternal device(s) 110 similar to the above description for IMD 101.

System 100 may include one or more additional IMDs, such as IMD 103,that may be implanted in various locations of patient 102 outside theventricles of heart 104 of patient 102. IMD 101 is illustrative of oneor more implanted devices, such as one or more implantable monitoringdevice, an implantable hub device, or implantable loop recorder.

IMD 103 as shown in FIG. 1, may comprise an implantable pressure sensingdevice that may be implanted within pulmonary artery of the patient. Insome examples, the pulmonary artery may comprise a left pulmonaryartery, whereas in other examples, pulmonary artery may comprise a rightpulmonary artery. For the sake of clarity, a fixation assembly for IMD103 is not depicted in FIG. 1.

As illustrated in FIG. 1, IMD 103 may be implanted, as one example,within a pulmonary artery of patient 102, and may include pressuresensing circuitry configured to measure the cardiovascular pressurewithin the pulmonary artery of patient 102. In some examples, IMD 103may include wireless communication circuitry, e.g., TCC and/or RFtelemetry circuitry, configured to receive a trigger signal from IMD 101and/or IMD 105, at electrodes or an antenna provided in IMD 103 (e.g.,an antenna such as one of the examples described in this disclosure).The pressure sensing circuitry of IMD 103 may be configured to measurethe cardiovascular pressure of patient 102 in response to receiving thetrigger signal. In either case, IMD 103 may be configured to transmitthe measured pressure values to IMD 101 and/or IMD 105 by wirelesscommunication. For example, IMD 103 may transmit measurements and dataacquired by IMD 103 related to pulmonary artery pressure and otherinformation generated by IMD 103 to IMD 101, to IMD 105, and/or toexternal device(s) 110. In various examples, IMD 103 comprises anantenna used for communications between IMD 103 and other devices ofsystem 100, arranged using the examples of antennas described throughoutthis disclosure, or any equivalents thereof.

For the remainder of the disclosure, a general reference to a medicaldevice system may refer collectively to include any examples of medicaldevice system 100, as described above with respect to FIG. 1, and anyequivalents thereof. Further, for the remainder of the disclosure ageneral reference to an IMD may refer collectively to include anyexamples of IMD 101, IMD 103, and/or IMD 105, as described above withrespect to FIG. 1, and any equivalents thereof.

The example IMDs of FIG. 1 include a housing configured to house atleast one of stimulation and sensing circuitry within an internal sideof the housing. For example, a battery such as lithium/iodine cell iscoupled to a motherboard that hosts one or more semiconductor chips andother electronic circuitry such as stimulation and sensing circuitry forproviding stimulation to patient 104 and sensing signals (e.g.,pressure, electrical, etc.) within patient 104. In some examples, thestimulation and sensing circuitry may be part of the one or moresemiconductor chips.

The motherboard and the battery are encased in a housing of the IMDs. Asone example, the housing may be formed with a metal cup that holds thebattery and other integrated circuitry and a wafer (e.g., anon-conductive wafer made from glass, sapphire, or other material) thatbonds to the metal cup. In some examples, the metal cup may be formedusing titanium or a titanium alloy, as two non-limiting examples. As anexample method of manufacturing, the stimulation and sensing circuitryare inserted into a metal casing, which may be in multiple pieces. Themultiple pieces are hermetically sealed with the wafer to together toform a housing configured to house at least one of stimulation andsensing circuitry within an internal side (e.g., inside) of the housing.

In one or more examples described in this disclosure, an antenna havinga curved (e.g., closed or partly open) structure may be coupled to anexternal side of the housing. For example, a thin layer of insulation(e.g., less than 1 mm such as 0.5 mm) is placed on the housing and theantenna having the curved structure is formed on the thin layer ofinsulation. In one example, the wafer is the thin layer of insulation.Also, the thickness of the insulation may be less than 0.5 mm, such as0.1 mm, and may be based on the desired mechanical strength of the layerof insulation.

As one example, the antenna is deposited on the outside (e.g., on thewafer) and conductive traces are deposited on the inside surface of thewafer and the components are arranged on the inside surface as well. Inone or more embodiments, the wafer can be a non-conductive or insulativesubstrate such that external contacts, the antenna, and any conductorsor other devices disposed on the wafer can be electrically isolated ifdesired. The wafer can include any suitable material or combination ofmaterials. The wafer (e.g., non-conductive wafer) can include at leastone of glass, quartz, silica, sapphire, silicon carbide, diamond,synthetic diamond, and gallium nitride, or alloys or combinations(including clad structures, laminates etc.) thereof.

Having the antenna with the curved (e.g., closed or partially open)structure on the outside of the housing may provide various advantages.For example, after implantation, the antenna is in contact with patienttissue (e.g., directly exposed to the patient tissue). The patienttissue tends to have a relatively high dielectric constant, and the sizeof the antenna is inversely proportional to the dielectric constant. Byplacing the antenna in contact with patient tissue, the size of theantenna can be reduced substantially as compared to prior antennaarchitectures where the antenna is formed within a polymer having muchlower dielectric constant.

As described above, in this disclosure, “in contact” may refer to anantenna being surrounded by material having a dielectric constant set bythe patient tissue such that the resonant frequency of the antennas is afunction of a dielectric constant of the patient tissue. In exampleswhere the antennas are in direct contact with the skin, the resonantfrequency of the antennas is a function of the dielectric constant ofthe skin. Stated another way, the functionality and operationalcharacteristics of the antennas is based on a dielectric constant of thepatient tissue or blood or possibly skin. As one example, the patienttissue is muscle tissue. As another example, the IMD may be surroundedby blood, and blood has a similar dielectric constant as muscle tissue.In this disclosure, although patient tissue is discussed, the exampletechniques may be applicable to examples where the IMD is surrounded byblood or the antenna is in contact with skin (e.g., antenna of externaldevice 110 that placed on the skin of patient 102 to communicate withIMD 101). Accordingly, patient tissue may be considered a general termto refer to patient anatomy that surrounds the IMD such as muscle orblood or patient skin.

As one example, the equation to determine resonant frequency of exampleantennas described in this disclosure having the curved structure is0.5λ=0.5c₀/(f√{square root over (e_(r))}), where c₀ is the lightpropagation speed in a vacuum, f is the frequency, and e_(r) is therelative permittivity of tissue (e.g., dielectric constant). The valueof λ may be approximately one-half the width of the antenna. The exampleof the width is shown in FIGS. 2, 3, and 5.

The above equation to determine resonant frequency is an approximation,and numerical methods may be used to determine resonant frequency ratherthan an analytical equation. In other words, for a desired frequency f(e.g., 2.4 GHz), the above equation provides an approximation of thewidth of the curved antenna, but some level of testing and modifying maybe needed to achieve the right size and shape such as based on thedesign of the IMD. Because the antenna is in contact with the patienttissue, in one or more examples described in this disclosure, thedielectric constant for determining the resonant frequency of theantenna is a function of the dielectric constant of the tissuesurrounding the IMD.

In some examples, the dimensions of the antenna may be dependent uponthe type of communication (e.g., may be frequency dependent). In theexamples below that provide example dimensions of the device, may be forthe BTLE communication. However, as also described below, in someexamples, the example antennas may be wide range and provide goodbehavior over a wide range of frequencies. As noted above, thetechniques to determine the dimensions of the antenna may be through anumerical method of testing different lengths and widths to achieve thedesired resonant frequency for the antenna.

In some examples, the antenna may be in direct contact with the patienttissue. However, it may be possible that in some examples, a protectivecoating is applied to the antenna to protect the IMD. Even in such ascenario, the antenna may be considered as being “in contact” withpatient tissue because the dielectric constant of the tissue isdeterminative of the resonant frequency of the antenna. For example, theantenna may be formed with titanium which tends to not corrode, but acoating of parylene or like substance may be used.

Antennas in prior antenna architectures may not be exposed directly tothe tissue because the high conductivity of the tissue causes highlosses in the radiation. For example, in prior antenna architectures, anoscillating current through the antenna causes the electromagnetic fieldto radiate. However, if the antenna is exposed to tissue and the housingis exposed to tissue, there is a low impedance path for the current fromthe antenna to the housing, which forms a ground. Therefore, rather thanthe current flowing through the antenna causing the electromagneticfield, a large percentage of the current flows to through the patienttissue, reducing the amplitude of the electromagnetic field that isradiated.

In these prior architectures, to avoid the antenna from being exposed totissue, the antenna is embedded in low dielectric material body (e.g.,called header). Due to the integration cost reason, often low dielectricmaterials such as a polymer (with dielectric constant normally 2˜4) areused to insulate the antenna. Since the antenna size is inverselyproportional the dielectric constant, with low dielectric constant, theantenna size is very hard to reduce, which increases the overall medicaldevice size. Also, due to the high loss of human tissue, the insulationbody is often required to be over a certain thickness, which furtherincreases the overall medical device size. Further, to make the antennaefficient, the antenna needs to be away from the metal shield (e.g.,housing is often called ‘can’ or ‘case’), as well as other metalcomponents in the header (such as the leads bore), which furtherincreases the device size. Furthermore, because of the large differencesin dielectric constant between the surrounding body tissue and theantenna insulation materials, the antenna impedance usually results innon-ideal number, therefore, adding a matching circuit to help theimpedance match is very common in medical device design, which increasesthe overall complexity of the device and introduces additional powerloss (i.e., reduction of device longevity).

In the examples described in this disclosure, because the antenna is onan external side of the IMD and not in a housing, the dielectricconstant is based on the patient tissue which is much larger than wherethe antenna is embedded in polymer of a header. Therefore, the examplesof the antenna described in this disclosure tend to be smaller,resulting in a smaller overall IMD. Furthermore, the antennas may beplanar (e.g., with little thickness or volume), and therefore, even whencoupled to an external side of the housing, do not increase the overallsize of the IMD.

Because the antenna is coupled to the external side of the IMD (e.g., onthe wafer), the antenna may be stacked on the wafer (e.g., the wafersuch as sapphire is less than 1 mm) as compared to other priorarchitectures. In some examples, the wafer may be the insulation and noadditional insulation is needed. In the prior architectures, if theantenna were proximate to the housing (or ground plane), the antennawould not radiate well since the current flowing in ground is in theopposite direction to the current through antenna, which results in acancellation of radiation. The reason is that when the insulation layerdielectric constant is much lower than that of the surrounding tissue,where the antenna is in contact with, the ground cancelling effect issignificantly reduced. For example, when the insulation thickness isonly 0.5 mm, the antenna design in this disclosure still performscomparable to normal antenna design in polymer header. An example of theantenna stacked on an IMD is illustrated with respect to FIG. 9.

In the antenna architecture of this disclosure, the antenna isself-contained and there is no need for an additional ground for thereto be a complete circuit. For example, the antenna forms a completeself-contained loop between one feed through point and ground or betweenthe two feed through points. Accordingly, the current flows from onefeed through point to the other feed point or ground rather than throughthe tissue. There may be some amount of current that flows from tissuebetween the feed through points. However, the antenna path may be oflower resistance than the tissue and therefore, there may be some lossin the amplitude of the electromagnetic wave, but the loss may beminimal.

FIG. 2 is a diagram of an implantable medical device in accordance withone or more examples described in this disclosure. For example, FIG. 2illustrates an example of IMD 200, which is an example of IMD 101. Asone example, IMD 200 has a length less than 50 millimeters (mm), a widthless than 10 mm, and a height less than 5 mm. As one example, length is45 mm, the width is 7.9 mm, and the height is 4.2 mm.

In some examples, the total volume of the IMD 200 may be less than 1500mm³, and the length, width, and height may be selectable to achieve thevolume. The above dimensions are used as an example and should not beconsidered limiting. The example techniques described in this disclosuremay be applicable to other types of medical devices.

IMD 200 includes housing 201. Housing 201 includes two parts. A firstpart is made from metal and forms a cup-like shape in which componentsof IMD 200 reside. A second part is a wafer that is bonded to the top tothe top of the metal to enclose IMD 200. The wafer may be formed with atleast one of glass, quartz, silica, sapphire, silicon carbide, diamond,synthetic diamond, and gallium nitride, or alloys or combinations(including clad structures, laminates etc.) thereof. In general, thewafer may be insulative (non-conductive). The metal part of housing 201may be formed with titanium or a titanium alloy. Other example metalsare possible (e.g., biocompatible metals), and if fully insulated fromdirect contact with tissue, then common metals, such as copper, may bepossible.

Housing 201 includes in an internal side (e.g., the volume insidehousing 201) and an external side (e.g., the exterior surface of thewafer that is in contact with tissue of patient 102 when IMD 200 isimplanted). In the internal side, housing 201 includes circuitry such asstimulation and/or sensing circuity 224 to provide stimulation andsensing capabilities such as those described above. For example, tomanufacture IMD 200, the stimulation and/or sensing circuitry 224 may beencased within pieces of housing 201 (e.g., metal portion of housing201) and the pieces of housing 201 may be hermetically sealed (e.g., thewafer and the metal portion are hermetically sealed) to form housing 201that includes stimulation and/or sensing circuitry 224.

Stimulation and/or sensing circuitry 224 may include, in one example,one or more sense amplifiers, filters, rectifiers, threshold detectors,comparators, analog-to-digital converters (ADCs), switches or otheranalog or digital components. When IMD 200 is configured to sensecardiac signals, stimulation and/or sensing circuitry 224 may includeone or more sensing channels for acquiring cardiac electrical signalsfrom two or more electrodes coupled to the stimulation and/or sensingcircuitry 224. Each sensing channel may be configured to amplify, filterand rectify the cardiac electrical signal received from selectedelectrodes coupled to the respective sensing channel to improve thesignal quality for sensing cardiac events, e.g., R-waves and P-waves.

Stimulation and/or sensing circuitry 224 may also include, in someexamples, pulse generation circuitry for generating and deliveringelectrical stimulation therapy, such as pacing and/ordefibrillation/cardioversion therapy. The pulse generation circuitry mayinclude one or more capacitors, a charging circuit, transformer(s),switches and the like. Stimulation and/or sensing circuitry 224 mayinclude other components for sensing non-cardiac or cardiac signals,including accelerometers, pressure sensors, biomarker sensors (such asglucose or potassium sensors), or any other type of sensor. Stimulationand/or sensing circuitry 224 may also include other types of therapycircuitry for providing therapy in addition to or instead of electricalstimulation therapy, including drug therapy, non-cardiac stimulationtherapy, or any other type of therapy.

On the external side, housing 201 may include one or more electrodes228A and 228B (as one example) for delivery of stimulation and/or forsensing. Also, in some examples, wires or other connectors may extendfrom housing 201 to one or more leads having electrodes for stimulationand/or sensing.

IMD 200 is configured for wireless communication with external device(s)110 or other devices implanted within patient 102. For wirelesscommunication, IMD 200 includes antenna 202. For example, as illustratedin FIG. 2, antenna 202 includes a feeding structure that includes feedpoints 205, 206A, and 206B. Feed points 206A, 206B may be coupledtogether (e.g., shorted together). Accordingly, feed points 206A, 206Bmay be considered as a single feed point 206. In some examples, ratherthan having feed points 206A, 206B, it may be possible to have a singlefeed point 206.

As one example, one of feed points 205 or 206 is coupled tocommunication circuitry 226 within housing 201 and the other one of feedpoints 205 or 206 is coupled to ground (e.g., metal of housing 201).Communication circuity 226 may include a transmitter, a receiver, or maybe a transceiver. That is, communication circuitry 226 may provide forbi-directional communication (e.g., transmit and receive communication)or uni-directional communication (e.g., receive but not transmit data ortransmit by not receive data). By connecting feed points 206A, 206B toground, the example illustrated in FIG. 2 may form aground-signal-ground because feed point 205, between feed points 206A,206B, is coupled to communication circuitry 226. Theground-signal-ground configuration is one example. In the example whereone of feed points 205 or 206 is coupled to ground, feed points 205 or206 (e.g., 206A and 206B) is coupled to the metal portion of housing201. As another example, feed points 205 or 206 form differential feedpoints, in which neither feed points 205 or 206 are coupled to groundand both feed into communication circuitry 226 within housing 201.

Feed points 205 and/or 206 may be coupled to communication circuitry 226through housing 201, such as the wafer of housing 201. As one example,communication circuitry 226 may be coupled to a transmission line thatcouples to feed points 205 or 206. Communication circuitry 226 may beconfigured to output a current (e.g., modulated current) that flowsthrough antenna 202 and causes antenna 202 to radiate an electromagneticsignal carrying the data that IMD 200 is to transmit. For receiving, anelectromagnetic signal may cause a current on antenna 202 thatcommunication circuitry 226 receives and demodulates to determine thedata that is transmitted to IMD 200. As described above, communicationcircuitry 226 may be configured for uni-directional communication insome examples.

Antenna 202 may be configured to communicate in accordance one or morewireless communication protocols such as Bluetooth®, WiFi, or medicalimplant communication service (MICS). That is, antenna 202 may beconfigured to have a resonant frequency that is approximately equal tothe frequency used for one or more example communication protocols.Here, approximately refers to the resonant frequency of antenna 202being within the range of frequencies that conform to the examplecommunication protocols.

Antenna 202 may be configured to have a curved (e.g., closed orpartially open) structure. A curved structure may refer to an arbitraryantenna where the antenna forms a complete enclosed structure withconnected ends with a gap separating a portion of the antenna from otherportions of the antenna. Examples of a curved structure includerectangles, circles, triangles, or other fully enclosed polygons. Ingeneral, example antennas described in this disclosure may be anarbitrary curved structure. A dipole antenna or a monopole antenna arenot examples of a curved structure. A pad antenna is not an example of acurved structure.

Although antenna 202 has a curved structure, antenna 202 may bedifferent than conventional loop antennas. A loop antenna is formed aswires, rather than being stacked on housing 201, as illustrated in FIG.2 and described in more detail below. Also, in a loop antenna, thecurrent distribution is circular within the electrical wire. Forinstance, the current distribution in the loop antenna can be consideredas going “up” one half of the loop and going “down” the other half ofthe loop. This type of current distribution is considered as beingout-of-phase. However, in some examples, the current distribution ofantenna 202 is in-phase. One example of in-phase current distribution isillustrated in FIG. 4 with respect to the example antenna 300illustrated in FIG. 3. The in-phase current distribution of antenna 202may be similar and not like that of a loop antenna.

In accordance with one or more examples described in this disclosure andas illustrated in FIG. 2, antenna 202 is stacked on housing 201 (e.g.,stacked on the wafer portion). Antenna 202 being stacked may mean thatdimensions of antenna 202 are less than the dimensions of housing 201.For example, rather than examples of loop antennas whose wires looparound the perimeter of the IMD, antenna 202 is formed on-top-of housing201. For example, in some examples, during manufacturing, a thin layerof material (e.g., the wafer which may be less than 1 mm such as 0.5 mmand even 0.1 mm or less) is formed as part of housing 201 to enclosehousing 201. The thin layer of material may be insulative material suchas non-conductive material with low dielectric loss (e.g., polymer,sapphire, glass, quartz, ceramic, etc.), and antenna 202 may be formedon top of the thin layer of material.

As one example, the antenna 202 may be a planar antenna. A planarantenna may refer to antenna 202 having very little volume. Forinstance, the width of antenna 202 is shown by lines 208 and 216 and thelength of antenna is shown by lines 212 and 220. The width of antenna202 may be less than 7 mm (e.g., 6.4 mm) and the length of antenna 202may be less than 18 mm (e.g., 12 mm). In one or more examples, theheight of antenna 202 may be relatively small (e.g., the thickness ofthe metal) may be less than 50 microns. The above dimensions areprovided merely as one example, and may be different based on specificimplementation needs.

In some examples, the total area of antenna 202 may be less than 120 mm²(e.g., 115.2 mm²) (including the area of gap 204), and the length andwidth may be selectable to achieve the area. As described above, a gapmay separate portions of an antenna having a curved structure. Forexample, in FIG. 2, there is gap 204 separating portions of antenna 202.The area of gap 204 may be less than 60 mm² (e.g., 50 mm²). Accordingly,the area of antenna 202, excluding gap 204, is more than 60 mm² (e.g.,70 mm²).

As one example, antenna 202 includes a first portion defined by a length214 (e.g., 4 mm) and width 212 (e.g., 6.4 mm), a second portion,orthogonal to the first portion, defined by a length 208 (e.g., 18 mm)and a width 210 (e.g., 0.7 mm), a third portion, orthogonal to thesecond portion and parallel to the first portion, defined by a length222 (e.g., 4 mm) and a width 220 (e.g., 6.4 mm), and a fourth portion,orthogonal to the third portion and parallel with the second portion,defined by length 216 (e.g., 18 mm) and a width 218 (e.g., 0.7 mm).

The first portion may have a length less than 5 mm and a width less than8 mm. The second portion may have a length less than 20 mm and a widthless than 1 mm. The third portion may have a length less than 5 mm and awidth less than 8 mm. The fourth portion may have a length less than 20mm and a width less than 1 mm.

As one example, length 214 and 222 may be less than 1 cm and greaterthan 1 mm, and in some examples less than 1 mm. Width 210 and 218 may beless than 1 cm and greater than 1 mm, and in some examples less than 1mm. The exact sizes of length 214 and 22 and width 210 and 218 may be amatter of design choice and can be determined through a numerical methodof trial-and-error. Length 208 and 216 may be based on the exampleequation of 0.5λ=0.5c₀/(f√{square root over (e_(r))}), as describedabove. Width 212 and 220 may be based on impedance of the tissue that isto surround antenna 202. Width 212 and 220 may also be determined basedon trial and error techniques to provide the desired matching.

Gap 204 separates the first portion from the third portion and thesecond portion from the fourth portion. In some examples, as illustratedin FIG. 2, the portion encompassed by gap 204 may be the wafer ofhousing 201.

In some examples, the dimensions of the example portions may define theoperational characteristics of antenna 202. For instance, the size oflength 214 and 222 may define the resonant frequency of antenna 202.During manufacturing, the size of length 214 and 222 may be set so as toachieve the desired resonant frequency. An example of calculating theresonant frequency of antenna 202 may be based on numerical method ofapproximating the resonant frequency. The size of width 210 and 218 maydefine the impedance of antenna 202. For example, during manufacturing,width 210 and 218 may be controlled such that the impedance of antenna202 is similar to the impedance of tissue that will surround antenna 202after implantation. In this way, antenna 202 may be specifically formedto minimize reflection at the intersection (e.g., interface point) ofpatient tissue and antenna 202. Also, the impedance of antenna 202 maybe set by width 210 and 218 so as to minimize impedance differencesbetween the transmission lines extending from the transmit/receivecircuitry (e.g., minimize reflections at feeding points 205 and 206).

As described above, antenna 202 may be a planar antenna that is stackedwith housing 201. Accordingly, when IMD 200 is implanted within patient102, antenna 202 is in contact with the tissue of patient 102. Havingantenna 202 in contact with the tissue of patient 102 may be beneficialfor various reasons. For example, the resonant frequency of antenna 202is based on e_(r), the dielectric constant, where the dielectric is thematerial surrounding antenna 202. Size of antenna 202 is inverselyproportional to the square root of the dielectric constant. For example,to keep the resonant frequency the same, while reducing the size ofantenna 202, means that material surround antenna 202 needs to have ahigher dielectric constant. Therefore, having a larger dielectricconstant allows for a smaller sized antenna 202 as compared to a smallerdielectric constant, and having a smaller sized antenna 202 may bebeneficial to allow a smaller sized IMD 200.

The dielectric constant of patient tissue is approximately 30 to 80 fora resonant frequency of 100 MHz to several GHz. Some other examples, inwhich an antenna is not in contact with patient tissue (e.g., where theantenna is encased in a header that is hermetically sealed off frompatient tissue), the dielectric constant tends to be approximately 2 to4 and may be as large as 10. The dielectric constant, in examples wherethe antenna is in a hermetically sealed header, tends to besubstantially less than the examples where antenna 204 is in contactwith patient tissue. Therefore, the antennas in the hermetically sealedheaders may be larger than antenna 202, which can be an undesirablecharacteristic of antennas.

There may be various reasons why prior architectures of antennas cannotbe in contact with patient tissue and therefore are hermetically sealed.For example, in these prior architectures of antennas, rather than acurrent flowing through the antenna, the current may flow throughpatient tissue to ground, resulting in poor radiation. In the example ofantenna 204, a majority of the current output from feeding points 205and 206 (e.g., 206A, 206B) flows through antenna 204 rather than throughtissue. This is because the metal of antenna 202 provides a lowerimpedance path as compared to tissue. In the prior architectures ofantennas, the current path through the antenna to ground was a higherimpedance path than the impedance path to ground via patient tissue.Accordingly, antenna 202 may not be sensitive to the conductivity ofsurrounding tissue.

For example, some prior architectures of antennas include dipole,monopole, or loop antennas. Dipole or monopole antenna (includinganything based on dipole or monopole antennas) are mostly used forimplantable devices, and their performance reduces significantly ifdirectly contacted with tissue, because the tissue conductivity shortsthe antenna to ground. Common loop antennas may be slightly betterbecause magnetic field retains energy better in lossy tissue. However,when directly contacted with tissue, common loop antenna also suffersfrom poor impedance, and it is sensitive to the nearby grounding (ormetal housing).

In the examples described in this disclosure, with feeding points 205and 206 in the center, there is improvement of impedance. Also, with thein-phase current, as described below, as opposed to current flow of loopantenna, there may be less sensitivity to metal housing or ground, suchas with direct contact with tissue and using a relatively low dielectricinsulation layer.

As one example, antenna 202 does not require the metal of housing 201 asa ground and can therefore be considered as being “self-contained.”Also, because antenna 202 may be insensitive to tissue conductivity,antenna 202 is in contact with patient tissue without degradation inoperation since little to no current flows through the tissue.

In this manner, FIG. 2 illustrates an example of IMD 200 that includeshousing 201 configured to house communication circuitry 226, and in someexamples, at least one of stimulation and sensing circuitry 224 withinan internal side of housing 201, and antenna 202, which is a planarantenna, having a curved structure, that is stacked on an external sideof housing 201. In one or more examples, the resonant frequency ofantenna 202 is based on a dielectric constant of tissue surroundingantenna 202 when IMD 200 is implanted. For example, antenna 202 may bein contact with tissue when IMD 200 is implanted.

Antenna 202 may also exhibit various other characteristics that aredescribed in more detail below. For example, a current distribution ofantenna 202 may be in-phase in opposite sides of antenna 202 (e.g., thecurrent distribution in the first and third portions may be in-phase sothat current is flowing in the same direction in the first and thirdportions). Also, the feeding structure including feeding points 206A and206B may be located approximately in the center of antenna 202.Moreover, there may be additional feeding structures having respectivefeeding points that can be added to antenna 202.

FIG. 3 is a diagram illustrating an example of an antenna in accordancewith one or more examples described in this disclosure. FIG. 3illustrates antenna 300, which may be similar to antenna 202, but thefeeding structure having feeding points 302A and 302B are locateddifferently than feeding points 206A and 206B of FIG. 2. Antenna 300 maybe a planar antenna having a curved structure that can be stacked onhousing 201. As illustrated, feeding points 302A and 302B, which aresimilar to feeding points 206A and 206B, are located at the end ofprotrusions 316A and 316B. Feeding points 302A and 302B may be locatedanywhere along protrusions 316A and 316B, and in some examples, theremay be no protrusions 316A and 316B, as illustrated in FIG. 5.

In some examples, rather than being only one feeding structure thatincludes feeding points 302A and 302B, there may be additional feedingstructures, such as feeding points 320A and 320B. Feeding points 320Aand 320B are identified with dashed lines to indicate that feedingpoints 320A and 320B are optional. Using feeding points 320A and 320Ballows for polarization diversity. In some examples, feeding points 320Aand 320B are orthogonal to feeding points 302A and 302B.

As illustrated, in FIG. 3, antenna 300 includes a first portion definedby length 306 and width 314, a second portion, orthogonal to the firstportion, defined by length 304 and width 310, a third portion,orthogonal to the second portion and parallel to the first portion,defined by length 308 and width 314, and a fourth portion, orthogonal tothe first portion and the third portion and parallel to the secondportion, defined by length 304 and width 312. The dimensions of thefirst, second, third, and fourth portions may be same as the dimensionsof the first, second, third, and fourth portions described above withrespect to antenna 202. The first portion and the third portion may beconsidered as being on opposite sides of antenna 300, and the secondportion and the fourth portion may be considered as being on oppositesides of antenna 300.

Similar to FIG. 2, there is a gap 301 between the first, second, third,and fourth portions. The dimensions of gap 301 can be determined basedon the dimensions of the first, second, third, and fourth portions. Forexample, the length of gap 301 is approximately equal to length304—(length 306+Length 308). The width of gap 301 is approximately equalto width 314—(width 310+width 312).

Assume that L1 equals length 304, L2 equals the length of gap 301, andL3 equals length 306 or length 308. Also, assume that W1 equals width314, W2 equals the width of gap 301, and W3 equals width 310 or width312. As one example, L1 equals 18 mm, L2 equals 10 mm, and L3 equals 4mm. As one example, W1 equals 6.4 mm, W2 equals 5 mm, and W3 equals 0.7mm.

These are example dimensions for approximately 2 to 3 GHz resonancefrequency when implanted, with 2.5 GHz center frequency. L2 and W2 maydetermine the resonant frequency based on the dielectric constant of thetissue when implanted, L3 enhances the radiation, but can be reduced to1 mm. Accordingly, L1=L2+2*L3, and can range from approximately 18 mm(e.g., where L2 is 10 mm and L3 is 4 mm) to 12 mm (e.g., where L2 is 10mm and L3 is 1 mm). In some examples, L1 is less than 20 mm. W3 may alsobe modified. For example, W3 may be a minimum of 0.5 mm. Accordingly, W1(e.g., width 314) may be approximately 6.4 mm (e.g., where W2 is 5 mmand W3 is 0.7 mm) to 6 mm (e.g., where W2 is 5 mm and W3 is 0.5 mm).

FIG. 3 also illustrates a current distribution through antenna 300. Thecurrent distribution is shown in more detail in FIG. 4. For ease, inFIG. 3, current distribution through the first portion is shown witharrow 318 and current distribution through the third portion is shownwith arrow 322. As can be seen, the direction of arrow 318 and arrow 322is the same (e.g., pointing downwards), which means that the currentdistribution through the first portion and the current distributionthrough the third portion is in the same direction. When the currentdistribution through the first portion and the current distributionthrough the third portion are in the same direction, the currentdistributions can be considered as being in-phase.

The current distribution of antenna 300 may be different than that of aloop antenna. In a loop antenna, the direction of the currentdistribution through one portion of the loop antenna would be oppositeto the direction of the current distribution through an opposite portionof the loop antenna. For instance, the current can be considered aslooping through the antenna such that if the current is going up in oneportion it would be going down in the opposite portion (analogizing to aFerris wheel, at the top, the bucket will start to move down, and at thebottom (e.g., opposite to the top), the bucket will start to move up).

Accordingly, antenna 300 is an example of an antenna having a curvedstructure formed on an external side of the housing (e.g., housing 201).Antenna 202 may be similar to antenna 300. Also, a current distributionof antenna 300 is in-phase in opposite sides of antenna 300.

FIG. 4 is a conceptual diagram illustrating an example of currentdistribution on the example antenna illustrated in FIG. 2 or 3. At theresonant frequency, the current distribution is as shown in FIG. 4,where the antenna (e.g., antenna 300 used for illustration purposes) islocated inside the human tissue. For example, current distribution 400is the current distribution of antenna 202 or antenna 300.

As illustrated, most of the current are in sides 402A and 402B and thecenter line 402C. Sides 402A and 402B correspond to the first portionand the third portion of antenna 300. Center line 402C corresponds toprotrusions 316A and 316B. In the example illustrated in FIG. 4, thecurrent distribution through sides 402A, 402B, and 402C are all in thesame direction (e.g., in-phase). The in-phase current distribution mayresult in an enhanced radiation on the direction perpendicular to theantenna plane to outside the human body.

FIG. 5 is a diagram illustrating another example of an antenna inaccordance with one or more examples described in this disclosure. FIG.5 illustrates antenna 500, which may be similar to antenna 300 but doesnot include protrusions 316A and 316B. The dimensions of antenna 500 maybe similar to those of antenna 300 or antenna 202.

FIG. 6 is a graph illustrating an example of return loss of an antennain accordance with one or more examples described in this disclosure. Ingeneral, FIG. 6 illustrates the example performance of antennas likeantennas 202, 300, or 500. The example of FIG. 6 is generated by placingan example antenna in simulated tissue (e.g., artificial tissue havingsimilar characteristics of human tissue) and determining its electricalcharacteristics, like S11. S11 represents how much power is reflectedfrom the antenna, which indicates how much power was not radiated.

For example, FIG. 6 illustrates the amount of power that is reflected bythe antenna as a function of frequency. As illustrated, near thefrequencies of interest (e.g., 2.4 GHz for Bluetooth®), the S11 isapproximately −7.3 dB, which means that less than 20% of the power isreflected back, even without the use of a matching circuit, which issometimes used to match impedances. Such performance may be much betterthan normal antenna designs (monopole, dipole, or loop).

Moreover, FIG. 6 illustrates that the antenna has a relatively wide bandof operation. For example, the S11 is less than 6 dB between 2 to 3 GHz,which means that the example antennas described in this disclosure maybe useable for wide range of frequencies, where the amount of power thatis radiated is approximately 80% or greater.

Also, the example of FIG. 6 is tested using an example curved antennathat is stacked on the non-conductive wafer, with the wafer having athickness of less than 1 mm, for instance 05. mm. In other words, theS11 was measured in a condition where the antenna is less than 1 mmabove ground. Only 1 mm separating antenna from ground usually resultsin very narrow bandwidth. However, the example antennas described inthis disclosure show good S11 (e.g., return loss) over a wide frequencyrange.

FIG. 7 is a flowchart illustrating an example method of manufacturing inaccordance with one or more examples described in this disclosure. Partof manufacturing an IMD (e.g., IMD 200) may be forming housing 201 wherehousing 201 is configured to house at least one of stimulation andsensing circuitry (e.g., stimulation and/or sensing circuitry 224)within an internal side of housing 201 (700). For example, housing 201may be formed from a plurality of pieces and the stimulation and sensingcircuitry 224 may be encased by the pieces, and then housing 201 ishermetically sealed.

Part of manufacturing IMD 200 includes stacking a planar antenna (e.g.,antenna 202, 300, or 500), having a curved (e.g., closed or partly open)structure on an external side of housing 201 (702). For example, themanufacturing may be include forming a non-conductive wafer (e.g., withthickness less than 1 mm) that is bonded with a metallic cup, where thenon-conductive wafer forms the side of the housing 201. Antenna 202,300, or 500 is bonded to the external side of the non-conductive wafer(e.g., the portion that will be in contact with tissue).

In some examples, the manufacturing of IMD 200 may include forming afirst feeding structure with feeding points (e.g., 205, 206, 302A, 302B,502A, or 502B) and in some examples, forming a second feeding structurewith orthogonal feeding points (e.g., feeding points 320A and 320B).Forming the feeding structure(s) may include creating a connectionthrough housing 201 to transmission lines that couple the feeding pointsto stimulation and sensing circuitry 224 of IMD 200.

FIG. 8 is a block diagram illustrating an example of stackedarchitecture of an antenna on an implantable medical device. Asillustrated in FIG. 8, wafer 802 is formed part of the housing of IMD800. For instance, wafer 802 may be bonded to the metal cup and wafer802 and the metal cup together form the housing the houses components ofIMD 800.

IMD 800 may be any of the IMDs described above. In some examples, thethickness of wafer 802 may be less than 1 mm (e.g., 0.5 mm or 0.1 mm orless than 0.1 mm). Wafer 802 may be non-conductive material with lowdielectric loss such as polymer, sapphire, glass, quartz, ceramic, andthe like. Antenna 804 is formed on top of wafer 802. When implantedantenna 804 is exposed to tissue 806 (e.g., muscle or blood). Antenna804 may be any of the antennas described above.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware or any combination thereof. Forexample, various aspects of the described techniques may be implementedwithin one or more processors, including one or more microprocessors,digital signal processors (DSPs), application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), or any otherequivalent integrated or discrete logic circuitry, as well as anycombinations of such components. The term “processor” or “processingcircuitry” may generally refer to any of the foregoing logic circuitry,alone or in combination with other logic circuitry, or any otherequivalent circuitry. A control unit comprising hardware may alsoperform one or more of the techniques of this disclosure.

Such hardware, software, and firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components orintegrated within common or separate hardware or software components.

The techniques described in this disclosure may also be embodied orencoded in a computer-readable medium, such as a computer-readablestorage medium, containing instructions. Instructions embedded orencoded in a computer-readable storage medium may cause a programmableprocessor, or other processor, to perform the method, e.g., when theinstructions are executed. Computer readable storage media may includerandom access memory (RAM), read only memory (ROM), programmable readonly memory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, a hard disk, a CD-ROM, a floppy disk, a cassette, magneticmedia, optical media, or other computer readable media.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. An implantable medical device (IMD) comprising: ahousing configured to house communication circuitry within an internalside of the housing; and a planar antenna, having a curved structure,that is stacked on an external side of the housing and coupled to thecommunication circuitry, wherein the planar antenna is configured suchthat a current distribution of the planar antenna is in-phase inopposite sides of the antenna.
 2. The IMD of claim 1, wherein the planarantenna having the curved structure forms an enclosed structure withconnected ends.
 3. The IMD of claim 1, wherein a resonant frequency ofthe planar antenna is based on a dielectric constant of tissuesurrounding the planar antenna when the IMD is implanted.
 4. The IMD ofclaim 1, wherein the IMD is configured such that the planar antenna isin contact with tissue when the IMD is implanted.
 5. The IMD of claim 1,wherein the planar antenna comprises a feeding structure located atapproximately a center of the planar antenna.
 6. The IMD of claim 5,wherein the feeding structure comprises a first feeding structure, theplanar antenna further comprising a second feeding structure.
 7. The IMDof claim 1, wherein the planar antenna comprises a width less than orequal to approximately 7 millimeters (mm) and a length less than orequal to approximately 18 mm.
 8. The IMD of claim 1, wherein the housingcomprises a width of less than or equal to approximately 10 mm, a lengthless than or equal to approximately 45 mm, and a height less than orequal to approximately 5 mm.
 9. The IMD of claim 1, wherein the housingcomprises a non-conductive wafer, wherein the planar antenna is stackedon the non-conductive wafer, and wherein the planar antenna is notwithin a header formed on or coupled to the housing.
 10. The IMD ofclaim 1, wherein the planar antenna comprises a first portion having alength less than or equal to approximately 5 mm and width less than orequal to approximately 8 mm, a second portion that is orthogonal to thefirst portion having a length less than or equal to approximately 20 mmand width less than or equal to approximately 1 mm, a third portion thatis parallel with the first portion and orthogonal with the secondportion having a length less than or equal to approximately 5 mm andwidth less than or equal to approximately 8 mm, and a fourth portionthat is parallel to the second portion and orthogonal to the first andthird portions a length less than or equal to approximately 20 mm andwidth less than or equal to approximately 1 mm.
 11. The IMD of claim 1,further comprising at least one stimulation and sensing circuitry toprovide electrical stimulation or sense electrical signals through oneor more electrodes coupled to the IMD.
 12. An implantable medical device(IMD) comprising: a housing configured to house communication circuitrywithin an internal side of the housing; and an antenna having a curvedstructure formed on an external side of the housing and coupled to thecommunication circuitry, wherein a resonant frequency of the antenna isbased on a dielectric constant of tissue surrounding the antenna whenthe IMD is implanted, and wherein a current distribution of the antennais in-phase in opposite sides of the antenna.
 13. The IMD of claim 12,wherein the antenna is a planar antenna that is stacked on the externalside of the housing.
 14. The IMD of claim 12, wherein the IMD isconfigured such that the antenna is in contact with tissue when the IMDis implanted.
 15. The IMD of claim 12, wherein the antenna comprises afeeding structure located at approximately a center of the antenna. 16.The IMD of claim 15, wherein the feeding structure comprises a firstfeeding structure, the planar antenna further comprising a secondfeeding structure.
 17. The IMD of claim 12, wherein the planar antennacomprises a width less than or equal to approximately 7 millimeters (mm)and a length less than or equal to approximately 18 mm.
 18. The IMD ofclaim 12, wherein the housing comprises a width of less than or equal toapproximately 10 mm, a length less than or equal to approximately 45 mm,and a height less than or equal to approximately 5 mm.
 19. The IMD ofclaim 12, further comprising at least one stimulation and sensingcircuitry to provide electrical stimulation or sense electrical signalsthrough one or more electrodes coupled to the IMD.
 20. The IMD of claim12, wherein the housing comprises a non-conductive wafer, wherein theplanar antenna is stacked on the non-conductive wafer, and wherein theplanar antenna is not within a header formed on or coupled to thehousing.